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In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald (from the Anglo-Saxon manig [many] and feald [fold]) and refers to the folding together of multiple inputs and outputs (in contrast, an inlet or intake manifold supplies air to the cylinders).
Exhaust manifolds are generally simple cast iron or stainless steel units which collect engine exhaust gas from multiple cylinders and deliver it to the exhaust pipe. For many engines, there are aftermarket tubular exhaust manifolds known as headers in American English, as extractor manifolds in British and Australian English, [ citation needed ] These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers.and simply as "tubular manifolds" in British English.
The most common types of aftermarket headers are made of mild steel or stainless steel tubing for the primary tubes along with flat flanges and possibly a larger diameter collector made of a similar material as the primaries. They may be coated with a ceramic-type finish (sometimes both inside and outside), or painted with a heat-resistant finish, or bare. Chrome plated headers are available but these tend to blue after use. Polished stainless steel will also color (usually a yellow tint), but less than chrome in most cases.
Another form of modification used is to insulate a standard or aftermarket manifold. This decreases the amount of heat given off into the engine bay, therefore reducing the intake manifold temperature. There are a few types of thermal insulation but three are particularly common:
The goal of performance exhaust headers is mainly to decrease flow resistance (back pressure), and to increase the volumetric efficiency of an engine, resulting in a gain in power output. The processes occurring can be explained by the gas laws, specifically the ideal gas law and the combined gas law.
When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. When the exhaust valve opens, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an "exhaust pulse" comprising three main parts:
This relatively low pressure helps to extract all the combustion products from the cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as "scavenging". Length, cross-sectional area, and shaping of the exhaust ports and pipeworks influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.
The magnitude of the exhaust scavenging effect is a direct function of the velocity of the high and medium pressure components of the exhaust pulse. Performance headers work to increase the exhaust velocity as much as possible. One technique is tuned-length primary tubes. This technique attempts to time the occurrence of each exhaust pulse, to occur one after the other in succession while still in the exhaust system. The lower pressure tail of an exhaust pulse then serves to create a greater pressure difference between the high pressure head of the next exhaust pulse, thus increasing the velocity of that exhaust pulse. In V6 and V8 engines where there is more than one exhaust bank, "Y-pipes" and "X-pipes" work on the same principle of using the low pressure component of an exhaust pulse to increase the velocity of the next exhaust pulse.
Great care must be used when selecting the length and diameter of the primary tubes. Tubes that are too large will cause the exhaust gas to expand and slow down, decreasing the scavenging effect. Tubes that are too small will create exhaust flow resistance which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds.
Many headers are also resonance tuned, to utilize the low-pressure reflected wave rarefaction pulse which can help scavenging the combustion chamber during valve overlap. This pulse is created in all exhaust systems each time a change in density occurs, such as when exhaust merges into the collector. For clarification, the rarefaction pulse is the technical term for the same process that was described above in the "head, body, tail" description. By tuning the length of the primary tubes, usually by means of resonance tuning, the rarefaction pulse can be timed to coincide with the exact moment valve overlap occurs. Typically, long primary tubes resonate at a lower engine speed than short primary tubes.
Some modern exhaust headers are available with a ceramic coating. This coating serves to prohibit rust and to reduce the amount of heat radiated into the engine bay. The heat reduction will help prevent intake manifold heat soak, which will decrease the temperature of the air entering the engine.
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Crossplane V8 engines have a left and right bank each containing 4 cylinders. When the engine is running, pistons are firing according to the engine firing order. If a bank has two consecutive piston firings it will create a high pressure area in the exhaust pipe, because two exhaust pulses are moving through it close in time. As the two pulses move in the exhaust pipe they should encounter either an X or H pipe. When they encounter the pipe, part of the pulse diverts into the X-H pipe which lowers the total pressure by a small amount. The reason for this decrease in pressure is that the fluid (liquid, air or gas) will travel along a pipe and when it comes at a crossing the fluid will take the path of least resistance and some will bleed off, thus lowering the pressure slightly. Without an X-H pipe the flow of exhaust would be jerky or inconsistent, and the engine would not run at its highest efficiency. The double exhaust pulse would cause part of the next exhaust pulse in that bank to not exit that cylinder completely and cause either a detonation (because of a lean air-fuel ratio (AFR)), or a misfire due to a rich AFR, depending on how much of the double pulse was left and what the mixture of that pulse was.
Today's understanding of exhaust systems and fluid dynamics has given rise to a number of mechanical improvements. One such improvement can be seen in the exhaust ultimate power valve ("EXUP") fitted to some Yamaha motorcycles. It constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine speed. This ensures good low to mid-range performance.
At low engine speeds the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal valve within the exhaust—the EXUP valve—at the point where the four primary pipes from the cylinders join. This junction point essentially behaves as an artificial atmosphere, hence the alteration of the pressure at this point controls the behavior of reflected waves at this sudden increase in area discontinuity. Closing the valve increases the local pressure, thus inducing the formation of larger amplitude negative reflected expansion waves. This enhances low speed torque up to a speed at which the loss due to increased back pressure outweighs the EXUP tuning effect. At higher speeds the EXUP valve is fully opened and the exhaust is allowed to flow freely.
A turbocharger, colloquially known as a turbo, is a turbine-driven, forced induction device that increases an internal combustion engine's power output by forcing extra compressed air into the combustion chamber. This improvement over a naturally aspirated engine's power output is because the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure alone.
A carburetor or carburettor is a device that mixes air and fuel for internal combustion engines in an appropriate air–fuel ratio for combustion. The term is sometimes colloquially shortened to carb in the UK and North America or to carby in Australia. To carburate or carburete means to mix the air and fuel or to equip with a carburetor for that purpose.
A pulsejet engine is a type of jet engine in which combustion occurs in pulses. A pulsejet engine can be made with few or no moving parts, and is capable of running statically.
A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. This is in contrast to a "four-stroke engine", which requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NO
x) emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. NO
x is produced in high temperature mixtures of atmospheric nitrogen and oxygen that occur in the combustion cylinder, and this usually occurs at cylinder peak pressure. Another primary benefit of external EGR valves on a spark ignition engine is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses.
A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket. In most engines, the head also provides space for the passages that feed air and fuel to the cylinder, and that allow the exhaust to escape. The head can also be a place to mount the valves, spark plugs, and fuel injectors.
Engine tuning is the adjustment or modification of the internal combustion engine or Engine Control Unit (ECU) to yield optimal performance and increase the engine's power output, economy, or durability. These goals may be mutually exclusive; an engine may be de-tuned with respect to output power in exchange for better economy or longer engine life due to lessened stress on engine components.
In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. The word manifold comes from the Old English word manigfeald and refers to the multiplying of one (pipe) into many.
On a two-stroke engine, an expansion chamber or tuned pipe is a tuned exhaust system used to enhance its power output by improving its volumetric efficiency.
An exhaust system is used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system design, the exhaust gas may flow through one or more of:
A ram-air intake is any intake design which uses the dynamic air pressure created by vehicle motion to increase the static air pressure inside of the intake manifold on an internal combustion engine, thus allowing a greater massflow through the engine and hence increasing engine power.
A valveless pulsejet is the simplest known jet propulsion device. Valveless pulsejets are low in cost, light weight, powerful and easy to operate. They have all the advantages of conventional valved pulsejets, but without the reed valves that need frequent replacement - a valveless pulsejet can operate for its entire useful life with practically zero maintenance. They have been used to power model aircraft, experimental go-karts, and unmanned military aircraft such as cruise missiles and target drones.
A throttle is the mechanism by which fluid flow is managed by constriction or obstruction.
In engine technology, a reverse-flow or non-crossflow cylinder head is one that locates the intake and exhaust ports on the same side of the engine. The gases can be thought to enter the cylinder head and then change direction in order to exit the head. This is in contrast to the crossflow cylinder head design.
A four-stroke power valve is a device fitted to four-stroke engines that constantly adjusts the internal diameter of the exhaust system to better suit engine speed. At low engine speeds the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large-amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal butterfly valve within the exhaust at the point where the primary pipes from the cylinders join. This junction point essentially behaves as an artificial atmosphere. The alteration of the pressure at this point controls the behavior of reflected waves at this sudden increase in area discontinuity. Closing the valve increases the local pressure, inducing the formation of larger-amplitude negative reflected expansion waves. A servo motor controlled by the ECU opens and shuts the valve. The valve goes from being almost fully closed at idle speed, through to fully open at higher engine speeds. This ensures superior low to mid-range performance, more linear power output and reduced exhaust noise levels while the valve is in its reduced opening position.
Scavenging is the process of replacing the exhaust gas in a cylinder of an internal combustion engine with the fresh air/fuel mixture for the next cycle. If scavenging is incomplete, the remaining exhaust gases can cause improper combustion for the next cycle, leading to reduced power output.
Exhaust heat management is the means of lessening the damaging or performance-robbing effects of internal combustion engine exhaust heat by preventing heat from escaping from the exhaust system and into the engine compartment on automobiles.
In an internal combustion engine, the geometry of the exhaust system can be optimised ("tuned") to maximise the power output of the engine. Tuned exhausts are designed so that reflected pressure waves arrive at the exhaust port at a particular time in the combustion cycle.
An internal combustion engine (ICE) is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into useful work. This replaced the external combustion engine for applications where weight or size of the engine is important.